[stella] Song Player

I managed to convert my beat player into a pattern-based song player
tonight. The demo I've put together uses 250 bytes for the song player
code and another 250 bytes for the song data. I guess 500 bytes isn't too
bad, but it's a short song.

It doesn't require you to do anything mid-screen. And it needs 12 bytes of
RAM, but 3 of those can be temp storage.